I don't have a betta yet but I'll be getting one when we move houses. I'm going to buy the 2.5 gallon tank with a heater and filter before the move so I have it all besides the fish when I get all moved in. We have a local Petco that takes outstanding care of the fish and other animals that I'm going to be buying from but I want to know what to look for in the betta before buying it. How can I tell if there's an illness present? How do I tell if the fins are healthy? I just want to make sure that my first betta is going to be a healthy and happy one. I don't want to have a sick betta as my first betta because I won't know how to properly treat it until it's too late.

Well, watch out for ripped up fins with edges that are black/bloody looking. Also any thing on them that resembles mold or fuzz. Make sure it is not laying at the bottom with it's fins clamped. And if it looks like it has a marble stuck in it's throat then that means it is bloated (any number of things can cause bloat).
Hmmm, what else?...

Make sure there arent any frayed fins, or anything like that. No abnormal white spots. I usually choose the one that look like they are interacting with the betta in the jar next to theirs. I like the agressice guys, so i choose the puffed up guys who are flaring.Also, no wierd things hanging off of it, could be an external parasite.The stomach shouldnt be too big, because it could be bloated or have an internal parasite.I think thats the most of it, im trying to think about what else it you should think about.

-fuzz/mold
-abnormally large eyes
-ripped/torn fins that look brown at the edges or look to be falling apart
-The lumps are a big one...
-Anything that looks like a parasite... they can be external (you will know when you see one) or internal- the fish's poop will be white. Also, if you've noticed the fish seems to be trying to cough something up, is trying to scratch its gills on something, or is shaking its head around a lot, it most likely has parasites in its gills.
-Any sores/bruises
-The bloat, which has been explained about
-Little white spots

...And that's all I can think of for now about the illnesses.

A healthy betta will flare at a neighbour or mirror, will be colorful, active, and males might even be working on a bubblenest.

most bettas in cups, unless they just got there or just got a water change, are usually gonna be laying on the bottom. o.0 Dante, my most active guy, was so bored and laying at the bottom, i couldn't even get him to flare at another betta. come to think of it.... out of all my males, only Gackt, Hyde, and Weiss were active and moving, and that's only because they'd just set Gackt up that day, Hyde was still in the shipping back, and Weiss has a bowl to move about in.

most bettas in cups, unless they just got there or just got a water change, are usually gonna be laying on the bottom. o.0 Dante, my most active guy, was so bored and laying at the bottom, i couldn't even get him to flare at another betta. come to think of it.... out of all my males, only Gackt, Hyde, and Weiss were active and moving, and that's only because they'd just set Gackt up that day, Hyde was still in the shipping back, and Weiss has a bowl to move about in.

Gackt and Hyde huh? I though I was the only one on here who likes them! My husband has a tattoo of L'arc en Ciel's Awake cd cover and my sister has Gackt's initials tattooed on her ankle (I just have the ever popular DBSK stuff tattooed all over me, if you have heard of them). :)